The focus of the research program is the synaptic control of the excitability of a retinal interneuron, the axon-bearing horizontal cell (HC1) of clawed frogs (Xenopus) and turtle retinas. In Xenopus, the HC1 is GABAergic and it receives direct synaptic input from green-sensitive rods, red-sensitive cones and a glycinergic interplexiform cell. Electrophysiological, pharmacological and morphological methods will be utilized to characterize (a) a GABA-sensitive HC1 to cone feedback synapse, (b) the non-linear summation of rod and cone inputs to the HC1 and (c) the influence of inner retinal neurons, acting through the glycinergic IPC, on rod-cone summation. In addition, we will characterize the GABA-sensitive channels in the cone synaptic base and the glycine-sensitive channels in the HC by voltage-clamp studies of solitary cells. In the turtle retinas, we seek to characterize the control of the electrical coupling among neighboring HC1 by a dopamine system. Electrophysiological and pharmacological studies will define the mechanisms controlling dopamine synthesis and release. Receptor binding studies seek to identify D1-type dopamine receptors on HC1 and D2-type dopamine receptors on a dopaminergic amacrine cell. The possibility that GABAergic amacrines contact dopaminergic amacrines in the inner plexiform layer will be studied by combined autoradiography and EM immunocytochemistry.